Vegetable oils usually used as food oils include soybean oil, rapeseed oil, cotton seed oil, safflower oil, corn germ oil, sunflower oil, rice bran oil and the like. In producing such vegetable oils, depending on the amount of oil contained therein, a raw material is pressed or the raw material is extracted with an organic solvent such as hexane to obtain miscella, and then the organic solvent is removed by evaporation from the miscella to yield a crude glyceride oil composition. Such a crude glyceride oil composition generally contains 0.5 to 10% by weight of impurities including phospholipid such as lecithin, etc., as main ingredient, waxes such as higher alcohols, etc., organic sulfur compounds, peptides, free fatty acids, hydrocarbons, carbohydrates, lower aldehydes, lower ketones, sterols, dye compounds and a small amount of metals, etc. These impurities are not desirable on quality of the products, because they cause polymerization or decomposition during preservation or on using or heating to result in oil coloration, generation of unpleasant odors and acceleration of oxidation or deterioration. It is necessary, therefore, to remove the gum materials, waxes and other impurities from the crude oil as much as possible.
Hitherto, in the oil industry, water is added to the crude oil to hydrate the gum material composed mainly of phospholipid, followed by swelling and coagulating the same to degum by centrifugal separation. Since the resulting degummed oil still contains about 0.2 to 1.0% by weight of gum material, it is usually subjected to chemical refining using chemicals such as alkali or acid, etc., to carry out removal of gum material and acid, namely, removal of mainly residual phospholipids and free fatty acids, followed by heating in vacuum together with an adsorbent such as activated clay, etc., to remove colors and other impurities such as heavy metals, free fatty acids, soaps or gum materials, etc., which cannot be removed by the above-described chemical refining. Further, it is generally processed in a dewaxing step for removing waxes and saturated tri- or diglycerides, etc., which crystallize or cause turbidity in the oil at a low temperature. Thereafter, unpleasant odor components such as lower aldehydes, ketones and free fatty acids, etc., are removed in the final step to obtain a purified glyceride oil having a gum content of 50 ppm or less as the final product.
However, the above-described prior purification process requires complicated chemical treatments involving chemical reactions except for the deodorizing step as the final purification step, and further it is desirable to obtain a purified glyceride oil suitable for food that the phospholipid content in the glyceride oil after the treatment for removing acids with alkalis is 100 ppm or less in the bleaching and deodorizing steps. Thus, in the prior art process, it is necessary to carry out repeatedly the gum removal operation. Consequently, not only a large amount of chemicals is required and a considerable amount of glyceride oil is lost, but at least a part of the glyceride oil deteriorates by various chemical treatments for removing gum material and acid to have a harmful influence upon the product glyceride oil and various secondary products obtained therefrom. Further, in order to carry out treatment for drainage which is remarkably polluted as the result of various chemical treatments or treatment for foots formed in the deacidification step, chemicals, equipment and expense are additionally required.
In order to remove such disadvantages, a novel process for purification of crude glyceride oil compositions was proposed in Japanese Patent Application (OPI) No. 153010/75 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"). In accordance with this process, after a crude glyceride oil composition is diluted with an organic solvent such as hexane, etc., it is brought into contact with an ultrafiltration membrane made of polysulfone, polyacrylonitrile or polyamide under pressure and the organic solvent is removed from a membrane permeable solution to obtain a degummed oil. However, according to this process, a removal rate to phospholipids in the crude glyceride oil composition is not sufficiently high because of characteristics of the ultrafiltration membrane, and, in the case of a crude glyceride oil composition containing several % by weight of gum material, it is difficult to reduce a gum material content in the degummed oil to 100 ppm or less which is the amount capable of effectively purifying so as to use for food by the above-described bleaching and deodorizing steps by one step membrane treatment described above. Thus, as described in Japanese Patent Application (OPI) No. 84206/77, an adsorption treatment using an expensive adsorbent such as alumina or silica is additionally required before or after the membrane treatment for miscella. As the result, technical and commercial advantages of the membrane treatment which is substituted for purification by chemical treatment are remarkably reduced. By the way, in case that the crude glyceride oil composition contains 2% by weight of gum material, the removal rate of the membrane for gum material should be 99.5% or more in order to reduce the gum material content in the resulting degummed oil to 100 ppm or less.
Further, in any of the above-described processes, since the ultrafiltration membrane used does not have sufficiently high resistance to glyceride oils and organic solvents for dilution and it easily softens at an elevated temperature, the molecular weight cut-off varies and removal ability for gum material is lost. Therefore, it is desirable that the membrane treatment is generally carried out at a comparatively low temperature of 10.degree. to 20.degree. C. As the result, since miscella having a comparatively high viscosity is subjected to membrane treatment, the amount of the permeable liquid is small and the treatment requires a long period of time. It is not preferred to reduce the glyceride concentration in the miscella, because the amount to be treated becomes large, though the viscosity reduces to increase the amount of the permeable liquid.